Typically, relatively large and heavy wires such as aluminum wire are used in wedge bonding on high power electronic packages. A bonding machine using wires with such large diameters for bonding generally comprises a bond head that is able to position itself relative to the electronic packages in the X, Y, Z and theta axes for wedge bonding. Having a theta axis is necessary in wedge bonding for aligning the bond orientation with the wire being fed between a first bond and a second bond.
Unlike thin wires, it is not possible to tear a large-sized wire away from the second bond solely by pulling it with a wire clamp after completing wire bonding at the second bond. The wire is usually cut partially by a cutter after the second bond is formed, and thereafter may be torn by the pulling movement of the bond head in a horizontal direction along the wire angle such that the tail of the cut wire remains under a bonding tool for subsequent bonding.
A conventional method of bonding and cutting large wires uses a flexible support such as a flexural disk to support a bonding tool. The bonding tool may include a transducer with a wedge and/or a wire clamp and a wire guide. A cutter is rigidly mounted to a bond head. When the bond head is oriented at the same direction as the orientation of the wire after a second bond is completed, the bond head is lowered further for cutting the wire. The wedge touches the wire with a preset force which does not damage the wire. Further downward movement of the bond head to allow the cutter to cut the wire can be achieved due to the flexible support provided by the flexural disk. Thus, the cutter moves together with the bond head to cut the wire with the wedge remaining relatively stationary. U.S. Pat. No. 6,439,448 entitled “Large Wire Bonder Head” discloses an example of such a bond head for a large wire bonder which adopts two circular flexural disks for providing flexible vertical movement of the bonding tool. For wire cutting, the flexural disks permit the bonding tool to touch the wire softly with a programmable force that is driven by a coil and magnet system. The cutter is pushed towards the wedge after bonding to cut the wire, and moves back to its original position after cutting the wire. The shortcoming of this apparatus is that the flexural disks result in the bonding tool having insufficient rigidity during wire bonding operations, which may lead to unstable bonds being formed.
It is also important that the cutter of a bond head is protected from hitting hard surfaces as it cuts the wire, such as the surface of the substrate on which wire bonding is performed. U.S. Pat. No. 6,827,248 entitled “Cutting Device for Bonded Wires” discloses a wire cutter which is elastically mounted onto a bond head such that the cutter can move upwards when the cutting force approaches a peak force limit. In this way, the cutter is protected from impact when it hits hard surfaces. However, the peak force limit varies with the material selected for providing elasticity. Wires of different types and sizes also require different peak force limits during cutting. Therefore, a universal design of the elastic mount for the cutter is not possible for cutting wires of different types and sizes.
It is further beneficial to be able to test the strength of bonds (either the first bond and/or the second bond) after bonding to check if the bonds formed are strong. U.S. Pat. No. 5,894,981 entitled “Integrated Pull Tester With An Ultrasonic Wire Bonder” discloses a wire bond head which is integrated with a pull tester for testing the strength of the bonds. The test is conducted by pulling the wire after bonding with a closed wire clamp or with a friction disk when the bond head moves upwards. The feeding length of the wire is detected by an encoder which determines whether the bond is defective. If the bond leaves the substrate such that no wire feeding is detected during the pulling test, the bond is defective. If a certain amount of wire feeding is detected, the bond is deemed good as the bond has adhered to the bond pad. However, detecting wire feeding during such pull test is not reliable as wires may elongate to different extents even for wires of the same material. In fact, the variation of the extent of wire elongation during the pull test is on a similar scale to the small amount of wire feeding detected for a good bond. This makes the said pull test unreliable as the supposed wire feeding may be due to wire elongation.
It is thus desirable to design a bond head for cutting large wires with precision while avoiding damage to the cutter. It is also useful to devise a reliable wire pull test.